1. Field of the Invention
The present invention relates to tumbler locks, more particularly, to a device for preventing a key from being inserted into a tumbler lock.
2. The Prior Art
Tumbler locks have a variety of uses, the most prevalent being as building door locks and as automotive ignition and door locks. There are occasions when it is desired to prevent a tumbler lock from being opened, even by the correct key. For example, a worker is given a key to a room that is only to be used during normal business hours. Or a person is loaned a car, but is told not to open the trunk. Or a person renting a locker is behind in his/her rental payments. In all cases, the key will be able to open the lock at any time, but for some reason, the person in possession of the key is to be prevented from doing so. A number of devices have been developed over the years to prevent the correct key from opening a lock, for example, U.S. Pat. No. 1,696,326 issued to Roethlisberger, U.S. Pat. No. 1,728,310 issued to Sundel, U.S. Pat. No. 2,068,936 issued to Unterberg, and U.S. Pat. No. 3,408,842 issued to Barnes et al. The common element of these disclosures is a plug that is inserted into the keyway that prevents any key from being completely inserted.
The plug of Roethlisberger fits into the keyway so that the outer tumblers return to rest on the lower arm of the plug, in front of the upper extension, blocking the plug from being removed. The major shortcoming of this design is that the plug must be long enough so that all of the tumblers drop onto the lower arm. If the arm is too short, the outermost tumblers will drop below the arm, to their normal locked position. If this should happen, the plug cannot be removed, even by the mating extractor, because the extractor cannot be inserted past the outermost tumbler. Another related shortcoming is that the inner end of the keyway must be at least partially closed so that the plug cannot be pushed too far and preventing the outermost tumbler from dropping as describe above.
The plug of Sundel solves the problems of Roethlisberger with an external plate that prevents the plug from being inserted too far into the keyway. However, unlike Roethlisberger, when the plug of Sundel is installed, it is very visible because to the external nature of the plate. Also, the external plate gives a person attempting to remove the plug a point of leverage, which may result in destruction of the lock.
The plug of Unterberg also attempts to solve the problem of being pushed to far into the keyway by using a notch in the upper surface of the plug that the innermost tumbler can fall into. However, because the notch must pass all of the other tumblers, it must be sloped on both sides. So it is still possible for the plug to be pushed too far into the keyway. Unterberg also uses a small external hook that prevents the plug from being inserted too far. However, the hook, like the external plate of Sundel, makes the plug more visible and gives a person leverage in an attempt to remove the plug, which would be relatively easy to do, since there is nothing holding the plug firmly in the keyway.
Unterberg and the plug of Barnes also have the problem that they, like Roethlisberger, must be long enough to extend over all of the tumblers. If the outermost tumbler should drop in front of the plug, the plug could not be removed. The flat end of the upper arm of the plug would be caught against the tumbler and the flat end of the extractor could not be inserted past the tumbler.